Socket Programming socket: a data structure containing...

Socket Programming

socket: a data structure containing connection information

Connection identifying information:

• client IP (Internet Protocol) address

• client port number

• source IP address

• source port number

Client-server connection:

• server creates a socket and listens for connections on a well-knownport number

• client creates a socket and connects to the server address at thewell-known port number

Sugih Jamin ([email protected])

TCP ConnectionWinSock API TCP Server

socket()

bind()

listen()

accept()

closesocket()

WSAStartup()WSAEnumProtocols()

WSACleanup()

blocks untilclient connects

WinSock API TCP Client

socket()

connect()


recv()

send()

processrequest

send()

recv()

connection establishment

data request

data reply

closesocket()

WSACleanup()


server.cint visits;intmain(int argc, char *argv[]){

struct sockaddr_in self, client;struct hostent *cp;int sd, td, len;char buf[BLEN];

sd = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP);memset((char *) &self, 0, sizeof(struct sockaddr_in));self.sin_family = AF_INET;self.sin_addr.s_addr = INADDR_ANY;self.sin_port = htons((u_short) PORT);bind(sd, (struct sockaddr *) &self, sizeof(struct sockaddr_in));listen(sd, QLEN);

while (1) {len = sizeof(struct sockaddr_in);td = accept(sd, (struct sockaddr *) &client, &len);cp = gethostbyaddr((char *) &client.sin_addr, sizeof(struct in_addr), AF_INET);printf("Connected from %s\n", cp->h_name);visits++;sprintf(buf, "This server has been contacted %d time(s).\n", visits);send(td, buf, strlen(buf), 0);close(td);

}}


client.cintmain(int argc, char *argv[]){

struct sockaddr_in server;struct hostent *sp;int sd;int n;char buf[BLEN];

sd = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP);

memset((char *) &server, 0, sizeof(struct sockaddr_in));server.sin_family = AF_INET;server.sin_port = htons((u_short) PORT);sp = gethostbyname(SERVER);memcpy(&server.sin_addr, sp->h_addr, sp->h_length);

connect(sd, (struct sockaddr *) &server, sizeof(struct sockaddr_in));

n = recv(sd, buf, sizeof(buf), 0);while (n > 0) {

write(1, buf, n);n = recv(sd, buf, sizeof(buf), 0);

}

close(sd);exit(0);

}


includes and defines

To be prepended to both server.c and client.c:

#include <stdio.h>#include <stdlib.h>#include <string.h>#include <unistd.h>#include <sys/types.h>#include <sys/socket.h>#include <netinet/in.h>#include <arpa/inet.h>#include <netdb.h>

#define SERVER "localhost"#define PORT 4897#define BLEN 256#define QLEN 200


Socket APIs Highlights

WinSock APIs:

socket: creates a socket data structure

Then we need to populate the structure with the connection identifyinginformation:

• client IP (Internet Protocol) address

• client port number

• source IP address

• source port number


TCP Socket Addresses

In the socket structure:

match incomingpkts’ destination

copy to outgoingpkts’ destination

bind()

connect()

IP address Port#

bind: used by server only, gives the server socket an IP address and/orport#

connect:

• TCP: initiates connection

• udp: remembers remote address


TCP Socket Addresses

TCP Server:IP address Port#

INADDR_ANY well-known

client’s address ephem-eral

TCP Client:IP address Port#

server’s address

client’s address ephem-eral

well-known


NAT and Firewalls

What are NAT (Network Address Translation) and firewalls?


NAT and Firewalls

NAT boxes remap port numbers (Why?)

A1 3

Ethernet LAN

N1 2 3 4

The Internet

port# mapping123

A.1A.3B.2

B

2 5

R

2 5

Firewalls may filter out all unknown ports and all UDP packets


Socket APIs Hightlights (cont)

listen:

• specifies max # of pending TCP connections

• only useful for connection oriented services

• TCP SYN denial of service attack

accept:

• waits for client connection

• returns a connected socket (different from the listening socket)



send:

• returns how many bytes are actually sent

• must loop to make sure that all is sent(except for blocking I/O, see UNP Section 6.2)

What is blocking and non-blocking I/O?

Why do you want to use non-blocking I/O?


Different Types of I/O

Synchronous: blocks (puts process to sleep) until I/O is readyBy default operations on sockets are blocking

Waiting for I/O:

1. wait for device availability

2. wait for I/O completion


Non-blocking I/O

Non-blocking I/O: keeps on checking (polling) until device is available

• set socket non-blocking:int on = 1; ioctlsocket(socket, FIONBIO, &on);

• call select on non-blocking socket

Signal-driven I/O: process gets a signal when device is available

• use WSAAsyncSelect() for signals tied to a window

• or WSAEventSelect() for signals not tied to a window

Asynchronous I/O: process notified when I/O completed

• Not widely supported yet

(See UNP Section 6.2 for more info)



recv:

• returns how many bytes are received

• 0 if connection is closed, -1 on error

• if non-blocking: -1 if no data with errno set to EWOULDBLOCK

• must loop to make sure that all is received (in TCP case)

• How do you know you have received everything sent?fixed size (part of protocol definition), prior handshake


Select

select(maxfd, readset, writeset, acceptset, timeout)

• synchronous (blocking) I/O multiplexing

• maxfd is the maximum file descriptor number + 1,so if you have only one descriptor, number 5, maxfd is 6.

• descriptor sets provided as bit mask. UseFD ZERO, FD SET, FD ISSET, and FD CLR

to work with the descriptor sets

• (the fourth parameter is usually called the exceptset)


Select (cont)

select(maxfd, readset, writeset, acceptset, timeout)

• returns as soon as one of the specified socket is ready for I/O

• returns # of ready sockets, -1 on error, 0 if timed out and no device isready (what for?)


recv with select vs. Polling

Which of the following would you use? Why?

loop {

select(. . . , timeout);

recv();

} till done;

or:

loop {

sleep(seconds)

recv();

} till done;



closesocket:

• marks socket unusable

• actual tear down depends on TCPif bind() fails, check WSAGetLastError() for WSEADDRINUSE


Socket Options: getsockopt and setsockopt (UNP Ch. 7)

• SO REUSEADDR: allows server to restart or multiple servers to bind tothe same port with different IP addresses

• SO LINGER: whether close should return immediately or abortconnection or wait for termination

• SO RCVBUF and SO SNDBUF: set buffers sizes

• SO KEEPALIVE: server pings client periodically


UDP Socket Programming

Server must always call bind(), but not listen() nor accept().

Client doesn’t need to call connect().

Use sendto() instead of send().

However, connect() can still be used to tell the system to remember theremote address. Then send() instead of sendto() can be used.

Call either recv() or recvfrom() to recv. recvfrom() also returnsthe address of the client.

UDP packets have boundary, not a byte-stream as in TCP, so recv()

retrieves one message at a time, i.e. no need to call recv() in a loop.


UDP DatagramWinSock API UDP Server

socket()

bind()

closesocket()


WSACleanup()

blocks untildatagram received

WinSock API UDP Client

socket()


sendto()

processrequest

sendto()data request

data reply

closesocket()

WSACleanup()

recvfrom()

recvfrom()


UDP Socket Addresses

UDP Server:

match incomingpkts’ destination

To be filled inwith sender’s addr.by kernel

bind()

IP address Port#

239.4.8.9 9489

UDP Client:

To be filled in withhost’s IP addr. andephemeral port by kernel

copied to outgoingpkts’ destination

connect()

IP address Port#

239.4.8.9 9489


UDP with connect()WinSock API UDP Server

socket()

bind()

closesocket()


WSACleanup()

blocks untildatagram received

WinSock API UDP Client

socket()


sendto()

processrequest

send()data request

data reply

closesocket()

WSACleanup()

recvfrom()

recv()

connect()


Byte Ordering

Big-endian: Most Significant Byte (MSB) in low address (sent/arrives first)(Sun Sparc, HP-PA)

Little-endian: MSB in high address (sent/arrives later) (Intel x86, PS2)

PowerPC and Alpha can be set to either mode

MMORG servers and backend servers may live on big-endian machines


Byte Ordering (cont)

Actual Value 1:

00000000 00000001MSB LSB

sent withouthtons and ntohs

little endian

MSB

LSB

big endianvalue: 2^8

LSB

MSB

00000000

00000001

A+1

A

00000000

00000001

A+1

A

00000000

00000001


Byte Ordering (cont)

To ensure interoperability, ALWAYS translateshort, long, int

to (from) “network byte order” before (after) transmissionby using these macros:

htons(): host to network shorthtonl(): host to network longntohs(): network to host shortntohl(): network to host long


Protocol Layers

Physical

Data Link

Network

Transport

Session

Presentation

Application

socket API

Domain NameSystem (DNS)


Naming and Addressing

DNS (Domain Name System) name in ASCII string:irl.eecs.umich.edu

IP address written out in dotted-decimal (dd) ASCII string:141.213.8.193

IP address in 32-bit binary representation:10001101 11010101 00001000 11000001

Use names instead of addresses:symbolic, easier to remember, variable length string

But fixed-length address provides more efficient handling and fastercomparison, uses less memory and less bandwidth (bw)


Name and Adress Manipulation

Syscalls to map name to/from address:

• dns to b: gethostbyname

• b to dns: gethostbyaddress

and to change representation:

• dd to b: inet addr (inet aton)

• b to dd: inet ntoa

dns to dd: gethostbyname plus inet ntoa


Other useful functions:

• gethostname: returns DNS name of current host

• getsockname: returns IP address bound to socket (in binary) Usedwhen addr and/or port is not specified (INADDR ANY), to find out theactual addr and/or port used

• getpeername: returns IP address of peer (in binary)

Debugging Tools: use tcpdump to look at packets on the networkhttp://windump.polito.it/install/


tcpdump Output

% sudo tcpdump -i fxp0 host tailtcpdump: listening on fxp008:52:07.913485 irl.eecs.umich.edu.3465 > tail.eecs.umich.edu.ssh: S 1334090569:1334090569(0)08:52:07.913766 tail.eecs.umich.edu.ssh > irl.eecs.umich.edu.3465: S 1738389661:1738389661(0)08:52:07.913820 irl.eecs.umich.edu.3465 > tail.eecs.umich.edu.ssh: . ack 1 win 57920 <nop,nop,timestamp08:52:07.965499 tail.eecs.umich.edu.ssh > irl.eecs.umich.edu.3465: P 1:41(40) ack 1 win 5792008:52:07.965857 irl.eecs.umich.edu.3465 > tail.eecs.umich.edu.ssh: P 1:40(39) ack 41 win 5792008:52:07.966126 tail.eecs.umich.edu.ssh > irl.eecs.umich.edu.3465: . ack 40 win 57881 <nop,nop,timestamp08:52:07.966392 irl.eecs.umich.edu.3465 > tail.eecs.umich.edu.ssh: P 40:584(544) ack 41 win08:52:07.966842 tail.eecs.umich.edu.ssh > irl.eecs.umich.edu.3465: . ack 584 win 57376 <nop,nop,timestamp08:52:07.995417 tail.eecs.umich.edu.ssh > irl.eecs.umich.edu.3465: P 41:577(536) ack 584 win08:52:07.995842 irl.eecs.umich.edu.3465 > tail.eecs.umich.edu.ssh: P 584:608(24) ack 577 win08:52:07.996143 tail.eecs.umich.edu.ssh > irl.eecs.umich.edu.3465: . ack 608 win 57896 <nop,nop,timestamp08:52:08.053504 tail.eecs.umich.edu.ssh > irl.eecs.umich.edu.3465: P 577:1001(424) ack 60808:52:08.146672 irl.eecs.umich.edu.3465 > tail.eecs.umich.edu.ssh: . ack 1001 win 57920 <nop,nop,timestamp08:52:08.182531 irl.eecs.umich.edu.3465 > tail.eecs.umich.edu.ssh: P 608:1024(416) ack 100108:52:08.183112 tail.eecs.umich.edu.ssh > irl.eecs.umich.edu.3465: . ack 1024 win 57504 <nop,nop,timestamp08:52:08.566220 tail.eecs.umich.edu.ssh > irl.eecs.umich.edu.3465: P 1001:1929(928) ack 102408:52:08.656695 irl.eecs.umich.edu.3465 > tail.eecs.umich.edu.ssh: . ack 1929 win 57920 <nop,nop,timestamp08:52:08.755094 irl.eecs.umich.edu.3465 > tail.eecs.umich.edu.ssh: P 1024:1040(16) ack 192908:52:08.755369 tail.eecs.umich.edu.ssh > irl.eecs.umich.edu.3465: . ack 1040 win 57904 <nop,nop,timestamp08:52:08.755452 irl.eecs.umich.edu.3465 > tail.eecs.umich.edu.ssh: P 1040:1088(48) ack 192908:52:08.755683 tail.eecs.umich.edu.ssh > irl.eecs.umich.edu.3465: . ack 1088 win 57872 <nop,nop,timestamp08:52:08.756357 tail.eecs.umich.edu.ssh > irl.eecs.umich.edu.3465: P 1929:1977(48) ack 108808:52:08.756654 irl.eecs.umich.edu.3465 > tail.eecs.umich.edu.ssh: P 1088:1152(64) ack 1977


Sources

Stevens, R., Unix Network Programming, 2nd. or 3rd. ed., Prentice-Hall,2004. All you ever want to know about socket programming, even if you’reusing WinSock.

Mulholland & Hakala, Programming Multiplayer Games, WordwarePublishing, 2004. Useful mainly for the WinSock coverage.

Bettner, P. and Terrano, M., “1500 Archers on a 28.8: NetworkProgramming in Age of Empires and Beyond,” GDC 2001

Bernier, Y.W., “Latency Compensating Methods in Client/Server In-gameProtocol Design and Optimization,” GDC 2001

Rothschild, J., “Mpath: Technical Considerations,” Mpath Interactive, 1997


Sources (cont)

Spohn, D., “Internet Game Timeline,” 2003http://internetgames.about.com/library/weekly/aatimelinea.htmhttp://internetgames.about.com/cs/gamingnews/a/goty2003.htm

Bartle, R., “Early MUD History,” 1990http://www.mud.co.uk/richard/mudhist.htm

Ng, Y-S., “Internet Game Design,” Gamasutra, Aug. 1, 1997

Ng, Y-S., “Designing Fast Action Games for the Internet,” Gamasutra,Sept. 5, 1997

Rosedale and Ondrejka, “Enabling Player-Created Online Worlds withGrid Computing and Streaming,” Gamasutra, Sep. 18, 2003


Sources (cont)

Filstrup, B., Cronin, E., and Jamin, S., “An Evaluation of Cheat-ProofingMethods for Multiplayer Games,” NetGames 2002

Brockington and Greig, “Neverwinter Nights Client/Server Postmortem,”GDC 2003, Gamasutra Mar. 6, 2003

Isensee and Ganem, “Developing Online Console Games,” Gamasutra,Mar. 28, 2003

Palm, “The Birth of the Mobile MMOG,” Gamasutra, Sep. 19, 2003


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